Control of a surface condition of a wafer has recently attracted attention from the viewpoint of increasing a yield in manufacturing of a semiconductor device. In a semiconductor device manufacturing process, films of various materials are formed on a silicon wafer. Therefore, an unnecessary film(s) and surface roughness are formed in a peripheral portion of the wafer. These days it is common practice to transfer a wafer while holding only a peripheral portion of the wafer with an arm. With such a background, an unnecessary film, remaining on a peripheral portion of a wafer, may peel off during various processes and may adhere to a device formed on the wafer, resulting in reduced yield. In order to remove an unnecessary film from the peripheral portion of the wafer, a polishing apparatus is used to polish the peripheral portion of the wafer.
FIG. 15 is a schematic view of a conventional polishing apparatus. A wafer W to be polished includes a first silicon layer 201 having an exposed surface, a patterned layer 202 underlying the first silicon layer 201, and a second silicon layer 203 underlying the patterned layer 202. A polishing tape 205 for polishing the wafer W is pressed by a pressing member 208 against an edge portion of the wafer W. The pressing member 208 is coupled to an air cylinder 209, and a force that presses the polishing tape 205 against the wafer W is applied from the air cylinder 209 to the pressing member 208. A positioning member 211 is secured to a rod of the air cylinder 209, and the positioning member 211 and the pressing member 208 are moved together by the air cylinder 209. A stopper 212 is in contact with a lower surface of the positioning member 211. Thus, the movement of the pressing member 208 and the polishing tape 205 is restricted by the stopper 212. The stopper 212 is coupled to a ball-screw mechanism 215, which is configured to be capable of vertically moving the stopper 212 at a set speed.
A peripheral portion of the wafer W is polished in the following manner. While rotating the wafer W about its axis, a liquid (e.g., pure water) is supplied onto an upper surface of the wafer W. The air cylinder 209 exerts a constant pressing force on the pressing member 208, which in turn presses the polishing tape 205 against the edge portion of the wafer W. As shown in FIGS. 16A and 16B, during polishing of the wafer W, the stopper 212 is lowered at a constant speed by the ball-screw mechanism 215 while the positioning member 211 keeps in contact with the stopper 212. The polishing tape 205 is pressed against the edge portion of the wafer W by the gradually descending pressing member 208 to polish the edge portion of the wafer W at a constant polishing rate, thereby forming a stepped recess in the peripheral portion of the wafer W.
However, the polishing load applied to the pressing member 208 during polishing of the wafer W changes depending on a hardness of a surface layer of the wafer W. For example, the first and second silicon layers 201, 203 are softer than the patterned layer 202; therefore, a force transmitted from the positioning member 211 to the stopper 212 during polishing of the first and second silicon layers 201, 203 is larger than a force transmitted from the positioning member 211 to the stopper 212 during polishing of the patterned layer 202. Accordingly, the polishing load during polishing of the patterned layer 202 is higher than the polishing load during polishing of the first and second silicon layers 201, 203. Consequently, the polishing load may exceed an appropriate range. Furthermore, if a surface layer of the wafer W is too hard, the stopper 212 may separate from the positioning member 211, resulting in an excessive polishing load.